1. Field of the Invention
This invention relates generally to the field of remote sensors and sensing, and in particular, to integrated remote sensors and sensing.
2. Description of Related Art
Many problems prevent wide-temperature range impedance measurements in harsh environments. Prior art impedance measurement devices and methods require very high tolerance, very expensive precision voltage or current sources. Precision impedance measurement underpins many sensor readout systems. For example, RTDs, thermistors, and associated measurement schemes. The other common temperature measurement sensor type, thermocouples, also relies upon high-gain temperature-stable amplifiers with minimal input offset and gain errors. These devices and methods are described by David E. Holcomb and Timothy E. McKnight, xe2x80x9cTransducers for Temperature, Pressure, and Flowxe2x80x9d, Encyclopedia of Applied Physics, Vol. 22, Wiley-VCH, 1998, pp. 35-65.
Prior art circuits also require temperature stable, high-accuracy time base circuits. Prior art circuits do not directly generate digital data as an output, and are further subject to temperature related errors associated with analog to digital conversion methods. Prior art circuits are not filly monolithically compatible.
Impedance measurement is not limited to temperature measurement and is often required for applications such as oil condition monitoring.
In situ monitoring of high-temperature degraded engine oil condition with micro sensors is described by Hang-Sheng Lee, Simon S. Wang, Donald J. Smolenski, Michael B. Viola, and Edward E. Klusendorf, Sensors and Actuators B, Vol. 20, 1994, pp.49-54
A sensor for determining the water content of oil-in-water emulsion by specific admittance measurement is described by Fernando Garcia-Golding, Mario Giallorenzo, Noel Moreno, and Victor Chang, Sensors and Actuators A, Vol. 46-47, 1995, pp. 337-341.
U.S. Pat. No. 5,274,335xe2x80x94Wang, et al. describes oil sensor systems and methods of qualitatively determining oil type and condition, 1993.
U.S. Pat. No. 5,089,780xe2x80x94Megerle describes an oil quality monitor sensor and system.
U.S. Pat. No. 4,733,556xe2x80x94Meitzler et al. describes method and apparatus for sensing the condition of lubrication oil in an internal combustion engine.
U.S. Pat. No. 5,332,961xe2x80x94Hammerle describes a resistive oil quality sensor.
A capacitive oil deterioration sensor is described in xe2x80x9cA Capacitive Oil Deterioration Sensorxe2x80x9d, George S. Saloka and Allen H. Meitzler, Society of Automotive Engineering International Congress and Exposition, Detroit Mich., Feb. 25-Mar. 1, 1991, pp. 137-146
U.S. Pat. No. 5,260,667xe2x80x94Garcia-Golding et al. describes a method and apparatus for determining the percentage water content of an oil-in-water emulsion by specific admittance measurement.
Accordingly, there is a long-felt need for simpler, more reliable and less expensive methods and apparatus for wide-temperature range impedance measurements, particularly in remote environments.
The long-felt need for simpler, more reliable and less expensive methods and apparatus for wide-temperature range impedance measurements, particularly in harsh environments, is satisfied by a new temperature compensated impedance measurement method and apparatus in accordance with the inventive arrangements. The inventive arrangement is applicable to both resistance- and capacitance-based measurements.
Prior approaches to impedance measurement have relied on precision component values and high-accuracy readout electronics. The inventive arrangements employ a new approach, namely tracking the thermal drift of components through thermal matching the entire measurement circuit preferably on a single silicon substrate. Other drifts associated with harsh environments such as pressure and vibration sensitivities are also compensated for by monolithic construction. Monolithic construction also assures minimal component value mismatch as compared to non-monolithic implementation schemes. The result is a breakthrough beyond even the most recent developments in the field. Very small, inexpensive impedance measurement devices in accordance with the inventive arrangements are monolithically compatible and capable of operating over a wide temperature range within very harsh environments. Use in oil well drill bits is a suggested application.
More particularly, the inventive arrangements use two identical oscillator circuits, each having an output frequency dependent on both embedded and sensing devices, the sensing devices being resistive and/or capacitive. Gated counters produce a result that is dependent only on the value of the sensing element. Temperature induced measurement errors are compensated for by controlling the counting window of the sensing oscillator with a fixed-width counter whose counting frequency is dependent only on fixed passive elements with matching temperature coefficients and error sources as the sensing oscillator. Temperature coefficients associated with passive elements, variations in power supply voltages, dc offsets, and other temperature dependencies are eliminated by ratioing the outputs of the two gated counters to produce an output solely dependent on the passive characteristics of the sensor. All passive elements except for the sensor are either temperature stable, or track the corresponding elements in the compensation channel very precisely, as is possible with monolithic construction.
This method, and corresponding apparatus, have several advantages over the prior art, including simplicity, wide temperature range operation, harsh environment tolerance, voltage supply variation tolerance, monolithic compatibility and direct digital output. It is a special advantage that a measuring system in accordance with the inventive arrangements is monolithically compatible, because monolithic sensors can operate at very low power levels making it ideal for portable applications.
Moreover, the temperature stable, precision amplifiers, current sources, voltage sources, and time bases associated with prior art methods are not needed. Temperature stable, precision amplifiers are sensitive to temperature-related gain and input offset errors. A monolithic sensor, in accordance with the inventive arrangements, comprises: a reference channel including a reference oscillator and a reference counter driven by the reference oscillator; at least one sensing channel including a sensing oscillator and a sensing counter driven by the sensing oscillator; a semiconducting substrate, the reference channel and the sensing channels being formed integrally with the substrate and intimately nested with one another on the substrate, the reference channel and the sensing channels thereby having precisely matched component values and temperature coefficients; a frequency determining component of the sensing oscillator formed integrally with the substrate, the frequency determining component having an impedance parameter which varies with an environmental parameter to be measured by the sensor; and a gating control, responsive to an output signal generated by the reference channel, for terminating counting in the sensing channels at an output count, whereby the output count is indicative of the environmental parameter, and successive ones of the output counts are indicative of changes in the environmental parameter.
The output signal generated by the reference channel can be a counter overflow indicator signal. The output signal could also be a programmed counter value.
The frequency determining component can be a capacitor, resistor, or inductor.
The impedance parameter can be capacitance, resistance, or inductance.
One of the terminals of the frequency determining component can be integrally electrically grounded. This can be employed allowing a single wire connection to the sensor.
The sensor can comprise a plurality of the sensor channels coupled to the reference channel and operating in parallel with one another.
The sensor can further comprise a scaler for adjusting the reference and sensing oscillators responsive to an output signal of the sensing counter. The scaler can control a binary weighted resistance switching network. This allows a wide-range impedance measurement.
The reference oscillator and the sensing oscillator can be phase locked to one another and use the phase difference as the indicator of sensor impedance variance.
While monolithic mounting typically is the most convenient technique for component matching, all that is required to match component value drift is to intimately thermally nest the components.
A method for remote sensing, according to the inventive arrangements, comprises the steps of: precisely matching component values and temperature coefficients of a reference counting channel and at least one sensing counting channel by intimately nesting the channels with one another on a semiconductor substrate; integrally forming in the substrate a frequency determining component of the at least one sensing channel, the frequency determining component having an impedance parameter which can vary with an environmental parameter; exposing the substrate to a variable environmental parameter; counting with the reference channel at a predetermined frequency, and at the same time, counting with the at least one sensing channel at a frequency related to the varying environmental parameter; terminating counting of the active sensing channels when the reference channel reaches a predetermined count; and correlating a final count of the at least one sensing channel with the environmental parameter, whereby the variable environmental parameter can be precisely, indirectly measured.